Study of two strains of epidemic (European) typhus and two strains of endemic (American) typhus in laboratory animals has shown their essential identity. .The characteristic typhus brain lesions can be shown to occur in all four strains after subcutaneous inoculation and Rickettsiae-filled cells can be found in the scrotal sac in all four strains after intraperitoneal inoculations. A strain of European typhus, which for 8 years has yielded no obvious scrotal reaction in guinea pigs, afterwards gave rise to a periodic scrotal reaction of variable severity. At present it occupies an intermediate position in this respect between the recently isolated European strain and the American strains. Variations in the scrotal reaction in guinea pigs appear to be correlated with strain variations in virulence in the human host. The incubation period in guinea pigs after intraperitoneal inoculation depends largely on the number of Rickettsiae injected.